1. Field of the Invention
The present invention relates to flat and split yarn having excellent heat stability and mechanical strength, such as tensile strength, knot strength which are prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use.
2. Discussion of the Background
In recent years, plastics have been increasingly used as packing materials, and as agricultural, fishery and forestry industrial materials and the like, all of which are required to have corrosion resistance, weathering resistance, wear resistance, high strength and the like. On the other hand, the waste from the large amounts of plastics used as the above materials has the possibility of polluting rivers, oceans and soil, and cause a great social problem. The appearance of plastics having biodegradability is thus eagerly awaited in order %o prevent such pollution. For example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and a raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because polymeric material sufficient enough to obtain practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as industrial parts, automotive members, domestic articles and the like because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
Flat yarns and fabrics thereof as one of packaging materials and agricultural materials required to have corrosion resistance, high strength and the like mainly consist of a high density polyethylene resin and polypropylene resin from among the polyolefins. The flat yarn is used as tying string and bands and cloth formed by weaving the yarn is useful as a curing sheet, civil engineering sheet, rice and wheat bags, cement bags, packing material such as flexible containers or the like, and as primary base cloth for carpet. Polyethylene terephthalate, polypropylene terephthalate and the like are not used for producing the flat yarn because of unstable winding and yarn separation when forming, and large deterioration rate thereof.
Further, it is no exaggeration to say that polymeric polyesters (referring to polymeric polyesters having number-average molecular weights of at least 10,000) generally used for forming of the yarn which is used one of the packing materials and agricultural, fishery and forestry industrial materials are limited to polyethylene terephthalate, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Although there are cases of 2,6-naphthalenedicarboxylic acid being used instead of terephthalic acid, there are no reports of trials which resulted in providing the obtained polymers with biodegradability.
Therefore, it is safe to say that there has been no concept of trying to make the molding of flat and split yarn using biodegradable aliphatic polyesters in which aliphatic dicarboxylic acid was used practical.
One of the reasons why this application concept has not been thought of is felt to be that in spite of the required special molding conditions and physical properties for the above flat and split yarn, most of the above-mentioned aliphatic polyesters have melting points of 100.degree. C. or lower even if they are crystalline, and have poor heat stability when melted above that. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters show markedly poor values even when they are the same level of number-average molecular weight as the above-mentioned polyethylene terephthalete, so just conceiving that the molded articles having required strength and the like would be obtained was difficult.
Another reason seems to be that studies for improving the physical properties of the aliphatic polyesters by increasing their number-average molecular weights were not make sufficiently advanced because of their poor heat stability.
The object of the present invention is to provide flat and split yarn prepared by using the above-mentioned aliphatic polyesters as their components which have sufficient high molecular weights for practical use, have excellent mechanical properties represented by heat stability and tensile strength, and which may be decomposed by microorganisms and the like, i.e., are biodegradable as one means of waste disposal so that they may be easily disposed of after the use thereof.